Merge remote-tracking branch 'quintela/migration.next' into staging

# By Orit Wasserman
# Via Juan Quintela
* quintela/migration.next:
  Fix error message in migrate_set_capability HMP command
  Allow XBZRLE decoding without enabling the capability
  Fix example for query-migrate-capabilities
  Add XBZRLE testing
  Move XBZRLE encoding code to a separate file to allow testing